There are a few options and it really depends on how complex you want it to be. The simplest is to create a sharing user on the server, grant that user permission to a new folder and share the folder. PC users can then connect to the share using the credentials of the sharing user. The most complex is to create an Active Directory domain, create users in AD, join the PCs to the domain and set up sharing on the server using the AD credentials.

If you have more questions please ask, but do tell us if it's for a school assignment as this makes a difference to the answers.

How much computing knowledge do you have?
What do you want to do with the server, store user data, share program data, run email?
Do the users share data or do they need their own personal storage?
How do you intend to back the server up?